Base station, mobile station, communication system, and reordering method thereof

ABSTRACT

Disclosed is a reordering method for sending packets, to which numbers indicating sequence are attached, from a base station to a mobile station, and rearranging the packets in order of the sequence number in the mobile station. When a handover source base station transfers packets which have not been sent to the mobile station, out of packets received from a host station, to a handover target base station, and the handover target base station sends the packets to the mobile station, data is sent from the handover source base station to the mobile station to notify that transference of the packets has been executed at the execution of handover sequence. The mobile station executed, reordering of the received packets when the transference of the packets has been executed.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of Application PCT/JP2007/055800, which was filedon Mar. 22, 2007, now pending, the contents of which are herein whollyincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a base station, mobile station,communication system and reordering method thereof, and moreparticularly to a base station, mobile station, communication system andreordering method where packets, to which numbers indicating sequenceare attached, are sent from a base station to a mobile station, and thepackets are rearranged in order of the sequence number in the mobilestation.

CDMA service based on the third generation method has begun in mobilecommunication systems, but a next generation mobile communication system(LTE: Long Term Evolution) which allows even faster communication is nowunder consideration at 3GPP (Third Generation Partnership Project) (seeNon-patent Document 1). Here a decrease in transmission delay is a majorissue, in addition to an increase in transmission rate.

In order to increase the transmission rate and decrease the transmissiondelay, an attempt to speed up the handover processing in comparison withthe conventional systems, is being made in the LTE communication system.In the case of mobile communication, a base station that communicateswith a mobile station is switched to another base station according tothe receive state when the mobile station is moving duringcommunication. This is referred to as handover and a base station incommunication before handover is called a “handover source basestation”, and a base station to be in communication after handover iscalled a handover target base station. Therefore speeding up thehandover is absolutely necessary to implement high-speed/low delaycommunication. In the LTE communication system, which is based on apacket exchange system, hard handover is used for the handover. In thecase of hard handover, a line between a mobile station and a handovertarget base station is connected after a line between the mobile stationand a handover source base station is disconnected. Hard handover can beperformed quickly by obtaining system information of the handover targetbase station immediately before executing handover, but transmission ofthe user data is interrupted during handover.

This means that in order to decrease transmission delay, it is importantto decrease the transmission interruption state and to prevent loss ofpackets during the transmission interruption state. If packets aredropped during the transmission interruption state, the dropped packetsare recovered by end-to-end packet retransmission, so transmission delayincreases.

Therefore in the handover of the LTE communication system, standardizedis a method in which a handover source base station transfers at leastpackets, among the data including control information and packetsaddressed to the mobile station, to a handover target base station (seeNon-patent Document 2). However, whether the data is actuallytransferred or not depends on whether this function is installed.

FIG. 19 is a diagram depicting packet transference during handover. In(A) of FIG. 19, two base stations 1 a and 1 b are connected to a hoststation (e.g. access gateway) 2. A mobile station 4 exists in a cell 3 aof the base station 1 a, and is currently communicating with the basestation 1 a. In this state, if the mobile station 4 moves toward thebase station 1 b and enters a cell 3 b, as shown in (B) of FIG. 19,handover is executed, and the communication base station of the mobilestation switches from base station 1 a to base station 1 b. A basestation in communication before handover is called a “handover sourcebase station” (source base station), and a base station to be incommunication after handover is called a “handover target base station”(target base station).

The handover source base station 1 a stores packets, received from thehost station 2, in an internal buffer, and sequentially sends thepackets stored in the buffer to the mobile station 4. Because of this,when handover occurs, some stored packets remain in the buffer withoutbeing sent to the mobile station. In (B) of FIG. 19, packets n−2 to n,which were received before handover and are not yet sent to the mobilestation, exist in the buffer, and these packets must be sent from thehandover target base station 1 b to the mobile station 4 after handover.Therefore when the handover sequence is executed, the handover sourcebase station 1 a transfers (forwards) the packets n−2 to n to thehandover target base station 1 b (this is referred to as forwarding). Byusing this forwarding method, in which the handover target base station1 b sends these packets to the mobile station 4 immediately afterhandover, packet transmission is not interrupted. Hence end-to-endpacket retransmission is not required, and high-speed handover can beexecuted. The above mentioned n−2 to n are numbers which indicate asequence of packets (sequence number).

FIG. 20 is a diagram depicting a handover of an LTE communicationsystem, and FIG. 21 is a diagram depicting a handover procedure which iscurrently assumed in the LTE communication system.

The mobile station (UE)4 notifies the handover source base station 1 athat handover HO is necessary, using the Measurement Report (1.Measurement control).

The handover source base station 1 a decides a target base station 1 bbased on the content of the Measurement Report (2. HO decision), andsends the handover request to this handover target base station 1 b (3.HO request). HO means handover. At this time, the handover source basestation 1 a also sends information on the mobile station (e.g. mobilestation ID and QoS (Quality of Service) information to the handovertarget base station). The handover target base station 1 b executes callacceptance control based on this information (4. Call acceptancecontrol).

If the handover target base station 1 b admits acceptance of the mobilestation, it returns handover response to the handover source basestation (5. HO response). Then, the handover source base station 1 ainstructs handover to the mobile station 4 (6. HO instruction),thereafter starts transfer of data (packets) before or after this step(packet transfer: forwarding).

The mobile station 4 which received the handover instruction insuressynchronization with the handover target base station 1 b by L1/L2signaling (7. Synchronization insuring), and when the synchronization isinsured, the mobile station 4 sends the handover completion report tothe handover target base station 1 b (8. HO completion).

Thereby the handover target base station 1 b sends the handovercompletion report to the host station 2 (9. HO completion). The hoststation 2 which received the handover completion report switches thepacket transmission path from the handover source base station 1 a tothe handover target base station 1 b (10. Path switching), and returnsthe HO completion response to the handover target base station 1 b (11.HO completion response). By the HO completion response, the handovertarget base station 1 b notifies the handover source base station 1 athat the handover HO is completed (12. Resource release). Then the pathbetween the handover source base station 1 a and the host station 2 iscleared (13. Resource release).

If packet transference (forwarding) is generated during the execution ofthe above mentioned handover sequence, the transferred packets may bejumped over by packets which flow into the handover target base station1 b from the host station 2, and the sequence numbers may be out oforder. If the handover target base station 1 b transfers the packets tothe mobile station 4 without correcting the order, the mobile stationcannot receive the packets in correct order, whereby communicationquality deteriorates and high quality communication before and after thehandover cannot be implemented as a result.

Therefore in the LTE communication system, the packet sequenceconsistency between the base station and the mobile station ismaintained by the following method. FIG. 22 is a diagram depicting thepacket sequence consistency, where the handover target base station 1 bmaintains packet sequence consistency by transmitting the packetstransferred from the handover source base station 1 a with priority overthe packets received from the host station. In other words, packets n−5to n have been stored in the handover source base station 1 a beforehandover, and handover is then generated, so the packets n−5 to n−3 aretransferred to the handover target base station 1 b and stored in thebuffer BF. In the buffer BF of the handover target base station 1 b,packets n+1 and n+2, received from the host station, have also beenstored.

The handover target base station 1 b, which stores the transferredpackets n−5 to n−3 and the packets n+1 to n+2 that flowed into in thebuffer from the host station, sends the packets n−5 to n−3 transferredfrom the handover source base station 1 a first to the mobile station.Then if a delay exists in the transfer of the packets n−2 to n from thehandover source base station 1 a, the handover target base station 1 bsends the packets n+1 to n+2 to the mobile station. The mobile station 4executes processing to rearrange the received packets in the order ofsequence numbers (reordering).

FIG. 23 is a diagram depicting the reordering processing of the mobilestation. In FIG. 22, the mobile station 4 received the packets n−5 ton−3 in the order of sequence numbers, so the packets are sequentiallytransferred to the upper layer. At the point of receiving the packetsn+1 to n+2, however, packets n−2 to n have not yet arrived. Thereforethe mobile station 4 stores the packets n+1 to n+2 in the buffer BF1until receiving these packets n−2 to n which have not yet arrived, anddoes not transfer these packets n+1 to n+2 to the upper layer. When thepackets n−2 to n are received, the mobile station 4 transfers thesepackets and packets n+1 to n+2 sequentially to the upper layer. An upperlimit is normally set for the packet wait time, and is measured by atimer of the mobile station, for example.

As mentioned above, in the case of handover in the LTE communicationsystem, the essential technologies are the packet transfer (packetforwarding) from the handover source base station to the handover targetbase station and the packet reordering processing at the mobile station.The relationship of these functions will now be described in detail.

FIG. 24 is a diagram depicting a protocol configuration between themobile station and the network. Between the mobile station and thenetwork, at least a PDCP (Packet Data Convergence Protocol) layer, RLC(Radio Link Control) layer and lower layer (MAC layer/physical layerMAC/PHY) are installed. These layers are all installed in the mobilestation, but are not always installed in a station at the network side.In the case of the example in FIG. 25, the PDCP layer is installed inthe host station aGW2, and the RLC layer and the lower layer areinstalled in the base station 1. The system may be constructed such thatall of the PDCP layer, RLC layer and lower layer are installed in thebase station 1, and only simple functions, such as the packet routingfunction and the sequence number attaching function, are provided toaGW2.

In the case of the example in FIG. 25, data in the PDCP layer isexchanged between the mobile station 4 and aGW2, which is the hoststation, and data in the RLC layer is exchanged between the userterminal 4 and the base station 1.

In other words, data addressed to the mobile station flows from theupper layer (e.g. IP layer) to the PDCP layer first, and becomes PDCPSDU (Service Data Unit), and then header information (e.g. sequencenumbers in PDCP layer) is added, and the PDCP PDU (Protocol Data Unit)is created.

The PDCP PDU is routed to the RLC layer, and becomes RLC SDU, and thenheader information (e.g. sequence number of RLC layer) is added, and RLCPDU is created. The RLC PDU arrives at the RLC layer of the mobilestation via the processing in the lower layer. In the RLC layer, theheader is removed and RLC SDU is reconstructed, then in the PDCP layer,the header of the PDCP PDU is removed, and PDCP SDU is created androuted to the upper layer of the mobile station.

In this protocol configuration, in the LTE communication system, packettransfer is executed in RLC SDU units or in PDCP SDU units, andreordering is executed in PDCP PDU units. Since RLC SDU and PDCP PDU aresubstantially the same data, they are simply referred to as “packets” inthe present description, and it is assumed that a number of a packetdescribed here indicates a sequence number of a PDCP PDU, unlessotherwise specified.

FIG. 26 is a flow chart depicting an operation of the handover sourcebase station device during handover.

When the handover source base station 1 a receives the field strength ofreception from the mobile station 4 via Measurement Report (step 101),the handover source base station 1 a judges whether handover HO isnecessary (step 102), and returns to the beginning if handover isunnecessary.

If it is decided that handover is necessary, the handover source basestation 1 a decides the handover target base station 1 b based on thecontent of Measurement Report, and sends a handover request to thehandover target base station 1 b (step 103).

Then the handover source base station 1 a receives a handover responsewhich is sent from the handover target base station 1 b (step 104), andjudges whether data transference is executed (step 105). If thetransference of the packets which are not sent to the mobile station andremain in the buffer is not executed, the handover source base station 1a sends HO instruction to the mobile station, and erases these packets(step 106). If the transference of the packets which are not sent to themobile station and remain in the buffer is executed, on the other hand,the handover source base station 1 a sends HO instruction to the mobilestation, and forwards these packets to the handover target base station(step 107). The transference of the packets for a service which requiresreal-time processing, such as VoIP calling, is not executed, but isdiscarded. This is because discarding packets insures voice transmissionand reception without delays. The transference of the packets for aservice which requires high QoS is executed.

Then the handover source station 1 a receives a resource release messagewhich is sent from the handover target base station 1 b (step 108), andexecutes a resource release (step 109).

FIG. 27 is a flow chart depicting operation of the handover target basestation during handover.

When the handover target base station 1 b receives an HO request(including mobile station ID and QoS information) from the handoversource base station 1 a (step 121), the handover target base station 1 bperforms call acceptance control based on this information, and judgeswhether acceptance of the mobile station is admitted or not (step 122).If not admitted, the handover target base station 1 b performspost-processing (step 130), and ends handover control.

If acceptance of the mobile station, on the other hand, is admitted, thehandover target base station 1 b returns an HO response to the handoversource base station 1 a (step 123). Then the handover target basestation 1 b stores packets transferred from the handover source basestation 1 a in a buffer (step 124), and receives an HO completion reportfrom the mobile station 4 (step 125). When the HO completion report isreceived, the handover target base station 1 b sends the HO completionreport to the host station 2 (step 126). When the handover completionreport is received, the host station 2 switches the packet transmissionpath from the handover source base station 1 a to the handover targetbase station 1 b, and returns with the HO completion response to thehandover target base station 1 b(step 127). When the HO completionresponse is received from the host station 2, the handover target basestation 1 b starts sending packets forwarded from the handover targetbase station 1 b preferentially to the mobile station, and startssending packets received from the host station 2 to the mobile stationafter the above packets are sent (scheduling: step 128). The mobiletarget base station 1 b also sends the resource release to the handoversource base station 1 a in parallel with step 128 (step 129), andperforms post-processing (step 130), and ends handover control.

FIG. 28 is a flow chart depicting an operation of the mobile stationduring handover.

The mobile station 4 notifies the field strength of reception to thehandover source base station using Measurement Report (step 151). Themobile station 4 then waits for an HO instruction from the handoversource base station 1 a, and if received (step 152), the mobile station4 insures synchronization with the handover target base station 1 b byL1/L2 signaling (step 153), and sends the handover completion report tothe handover target base station 1 b when synchronization is insured(step 154), then if packets are received from the handover target basestation 1 b, the mobile station executes reordering processing (steps155 to 160).

In other words, when the lower layer packets are received from thehandover target base station 1 b, the control unit of the mobile stationconstructs RLC SDU, and transfers this RLC SDU to the reorder unit (step155). The reorder unit checks whether the sequence number is continuous(step 156), and transfers this RLC SDU to the upper layer as PDCP SDU ifthe sequence numbers are continuous without missing any numbers (step160). If the continuity of the sequence number of a RLC SDU ceases, onthe other hand, the reorder unit stores RLC SDU(=PDC PDU)(step 157), andthen checks whether the continuity of the sequence number is resumed bythe received RLC SDU or not (step 158). If the continuity of thesequence numbers is resumed by the received RLC SDU, the reorder unittransfers this RLC SDU to the upper layer as PDCP SDU, and thentransfers the stored RSC SDU(=PDCP PDU) to the upper layer (step 160).

In step 158, if the sequence numbers of the received RLC SDU are notcontinuous, the mobile station monitors whether a predetermined time haselapsed (step 159), and repeats the processing after step 157 if not, ortransfers the stored PDCP PDU to the upper layer if elapsed, even if thesequence numbers are not continuous (step 160).

To execute packet transfer during handover in the LTE communicationsystem, the following problem exists. That is, when handover is executedin the LTE communication system, the transference of the packetsdestined for the mobile station remaining in the handover source basestation 1 a is executed as mentioned above, thereby the packets aretransferred to the handover target base station (forwarding). However,whether the transference of the packets executed during handover dependson whether this transference function is installed in the handoversource base station.

As a result, even whether the handover source base station 1 a did notexecute packet transfer (packet forwarding), the mobile station 4, whichwas not notified, may judge that packet transfer was executed, and insuch a case, after the handover, the reorder management unit of themobile station must unnecessarily wait for the arrival of packets ofwhich sequence numbers are continuous, until a predetermined timeelapses. This results in that communication delays increase, andthroughput deteriorates, and high quality communication quality cannotbe maintained before and after the handover.

FIG. 29 shows an example of packets n−2 to n remaining in the handoversource base station 1 a. If these packets are not forwarded to thehandover target base station 1 b, the mobile station 4 mustunnecessarily wait for the arrival of these packets n−2 to n, which willnever be sent for a predetermined time after receiving packet n+1 fromthe handover target base station 1 b. As a result, a communication delayoccurs, and throughput of the entire system drops.

A first prior art that is available is a reassembling and reorderingdevice, which restores packets before fragmentation from fragmentedpackets, and corrects reversal of the packet arrival sequence to recoverthe original sequence (Patent Document 1). However, this prior artrelates to a reassembling and reordering device which restores packetsbefore fragmentation from the packets fragmented in GTP tunnel (GTP:GPRS Tunneling Protocol) of GPRS (General Packet Radio Service), andcorrects the reversal of the packet arrival sequence generated by thefragmentation and reassembles to the original sequence.

A second prior art that is available is a mobile communication systemwhich realizes high-speed packet data transmission without generatingdata loss during handover between base stations in high-speed packetcommunication (Patent Document 2). In this mobile communication system,when a handover is generated between base stations in conformity with acommunication state of a mobile station during high-speed packetcommunication between a base station and a mobile station, the handoversource base station transfers the packet data to the handover targetbase station (forwarding).

However, neither the first nor the second prior arts are for suppressingan increase in communication delays or the deterioration of throughputdue to the reordering of packets received from the handover target basestation.

In view of the foregoing, it is an object of the present invention thatthe mobile station need not execute reordering at the packets in orderof the sequence numbers even if they are not continuous, if the packetsremaining in the handover source base station are not forwarded to thehandover target base station (transference is not executed) whenhandover control is executed.

It is another object of the present invention that the mobile stationexecutes reordering of the packets in order of the sequence numbers ifthe packets remaining in the handover source base station were forwardedto the handover target base station (transference was executed) whenhandover control is executed.

[Patent Document 1] Japanese Patent Application Laid-Open No.JP2004-135076A[Patent Document 2] Japanese Patent Application Laid-Open No.JP2004-282652A

[Non-patent Document 1] 3GPP, “Requirements for Evolved UTRA (E-UTRA)and Evolved UTRAN (E-UTRAN),” TR25.913 V7.3.0, Release 7, March 2006[Non-patent Document 2] 3GPP, “Evolved Universal Terrestrial RadioAccess (E-UTRA) and Evolved Universal Terrestrial Radio Access Network(E-UTRAN),” TS36.300, Release 8, Vol. 4.0, January 2007 SUMMARY OF THEINVENTION Reordering Method

A first aspect of the present invention is a reordering method forsending packets, to which numbers indicating sequence are attached, froma base station to a mobile station, and rearranging the packets in orderof the sequence number in the mobile station. This reordering methodcomprises: a first step of sending data to notify that transference ofthe packets has been executed at the execution of handover sequence tothe mobile station when a handover source base station transfers packetswhich have not been sent to the mobile station, out of packets receivedfrom a host station, to a handover target base station, and the packetsare sent from the handover target base station to the mobile station; asecond step of sending data to notify that transference of the packetsis not executed at the execution of the handover sequence, to the mobilestation when the packets are not transferred to the handover target basestation; and a third step of executing reordering of the receive packetsat the mobile station when the transference of the packets has beenexecuted.

In the third step of the present invention, the reordering processingends when packets having continuous sequence numbers are not receivedeven if reordering processing of the received packets is executed for apredetermined time.

The reordering method of the present invention further comprises a stepof deciding whether the packets, which have not been sent to the mobilestation, are to be transferred to the handover target base station or tobe erased, by referring to communication state information of thehandover target base station that is received from the handover targetbase station at the execution of the handover sequence.

The reordering method of the present invention further comprises a stepof deciding whether packets, which have not been sent to the mobilestation, are to be transferred to the handover target base station or tobe erased, depending on the type of service.

If numbers to indicate the sequence of the packets are attached to thepackets in the base station, the numbers attached to the packets, whichhave not been sent from the handover source base station to the mobilestation, are notified to the handover target base station when thepackets are transferred to the handover target base station.

Communication System

A second aspect of the present invention is a communication system forsending packets to which numbers indicating sequence are attached, froma base station to a mobile station, and rearranging the packets in orderof the sequence number in the mobile station. This communication systemcomprises a handover source base station, a handover target base stationand a mobile station wherein (1) the handover source base stationhaving: a buffer for storing packets received from a host station; apacket transmission unit for sending the packets stored in the buffer tothe mobile station; and a control unit which sends data to notify thattransference of the packets has been executed to the mobile station whenthe handover source base station transfers the packets, which have notbeen sent to the mobile station, to the handover target base station atthe execution of handover sequence, and sends data to notify thattransference of the packets is not executed to the mobile station whenthe packets are not transferred to the handover target base station, (2)the handover target base station having: a buffer which stores packetsreceived from the handover source base station and packets received fromthe host station at the execution of handover sequence; a control unitwhich controls such that the packets received from the handover sourcebase station are sent to the mobile station preferentially; and atransmission unit which sends packets to the mobile station, and (3) themobile station having: a buffer which stores packets received from thebase station; and a control unit which executes reordering of thereceived packets or does not execute reordering of the received packetsbased on the data received from the handover source base station andindicating whether transference of the packets has been executed.

The control unit of the handover source base station decides whether thepackets, which have not been sent to the mobile station, are to betransferred to the handover target base station or to be erased byreferring to the communication state information of the handover targetbase station that is received from the handover target base station atthe execution of the handover sequence.

The control unit of the handover source base station decides whether thepackets, which have not been sent to the mobile station, are to betransferred to the handover target base station or to be eraseddepending on the type of service.

Base Station

A third aspect of the present invention is a base station in acommunication system for sending packets to which numbers indicatingsequence are attached, from a base station to a mobile station andrearranging the packets in order of the sequence number in the mobilestation, comprising: a buffer for storing packets received from a hoststation and packets received from a handover source base station at theexecution of handover sequence; a packet transmission unit which sendspackets stored in the buffer to the mobile station; a handover controlunit which decides whether packets, which have not been sent to themobile station, are to be transferred to a handover target base stationat the execution of handover sequence or not, transfers the packets tothe handover target base station and data to notify that transference ofthe packets has been executed to the mobile station if the packets areto be transferred to the handover target base station, and sends thedata to notify that transference of the packets is not executed to themobile station if the packets are not to be transferred to the handovertarget base station; and a transmission control unit which sends packetsreceived by the handover source base station to the mobile station withpriority over the packets received from the host station at theexecution of handover sequence.

Mobile Station

A fourth aspect of the present invention is a mobile station in acommunication system for sending packets to which numbers indicatingsequence are attached, from a base station to a mobile station andrearranging the packets in order of the sequence number in the mobilestation, comprising: a buffer for storing packets received from the basestation; and a control unit which decides whether reordering of thepackets received from a handover target base station is to be executedor not based on data received from a handover source base station at theexecution of the handover sequence and indicating whether transferenceof the packets has been executed. The control unit ends reorderingprocessing when packets having continuous numbers are not received evenif reordering processing of the received packets is executed for apredetermined time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting a first embodiment;

FIG. 2 shows format examples of HO response messages includingcommunication state information;

FIG. 3 is a block diagram of a base station;

FIG. 4 is a block diagram of a mobile station;

FIG. 5 is a flow chart depicting an operation of a handover target basestation according to the first embodiment;

FIG. 6 shows a format example of an HO instruction message;

FIG. 7 is a flow chart depicting an operation of a handover source basestation according to the first embodiment;

FIG. 8 is a flow chart depicting an operation of a mobile stationaccording to the first embodiment;

FIG. 9 is a flow chart depicting a reordering processing in the mobilestation;

FIG. 10 is a diagram depicting a second embodiment;

FIG. 11 is a flow chart depicting an operation of a handover source basestation according to the second embodiment;

FIG. 12 is a flow chart depicting an operation of a handover source basestation according to a variant form;

FIG. 13 is a flow chart depicting a reordering processing in a mobilestation according to a variant form;

FIG. 14 is a block diagram of a base station of which buffer managementunit has a simplified configuration without packet transfer functions;

FIG. 15 is a flow chart depicting an operation of a handover source basestation according to the third embodiment;

FIG. 16 is a system block diagram focusing on layers;

FIG. 17 is a diagram depicting a fourth embodiment;

FIG. 18 is a diagram depicting a handover procedure of the fourthembodiment;

FIG. 19 is a diagram depicting transference of the packets duringhandover;

FIG. 20 is a diagram depicting handover of an LTE communication system;

FIG. 21 is a diagram depicting a handover procedure which is currentlyassumed in the LTE communication system;

FIG. 22 is a diagram depicting packet sequence matching;

FIG. 23 is a diagram depicting reordering processing of a mobilestation;

FIG. 24 is a diagram depicting a protocol configuration between a mobilestation and network;

FIG. 25 is a diagram depicting a case when a PDCP layer is installed inaGW, which is a host station, and an RLC layer and a lower layer areinstalled in a base station;

FIG. 26 is a flow chart depicting an operation of the handover sourcebase station device during handover;

FIG. 27 is a flow chart depicting an operation of the handover targetbase station during handover;

FIG. 28 is a flow chart depicting an operation of the mobile stationduring handover; and

FIG. 29 is a diagram depicting a problem of prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Outline of Embodiments

The problem is solved by enabling a base station and a mobile station toexecute the following procedures.

Procedure 1: During handover, a handover source base station or ahandover target base station notifies transference executioninformation, on whether data transference was executed or not, to themobile station.Procedure 2: When the transference execution information is received,the mobile station judges whether data transference was executed basedon this information, and if data transference was not executed, themobile station does not reorder the arrived packets, and transfers thepackets immediately to the upper layer as received.

In the conventional method, the handover source base station does notnotify the transference execution information to the mobile stationduring handover. Therefore, even when data transference is not executed,thereby the packets are not forwarded from the handover source basestation to the handover target base station, the mobile station mustwait needlessly for these not-forwarded packets until a predeterminedtime elapses, which generates an increase in communication delays and adeterioration of throughput. However if the mobile station is notifiedthe transference execution information according to the method of thepresent invention, it can know whether the data transference is executedor not, and if the packets were not forwarded, reordering can bestopped, whereby unnecessary reordering attempt of packets can beprevented. As a result, communication quality before and after handovercan be maintained to be higher than a conventional method.

(B) First Embodiment

FIG. 1 is a diagram depicting a first embodiment, where a handoversource base station 11 a judges the possibility of packet transfer(packet forwarding) based on the communication state of a handovertarget base station 11 b when the handover sequence is executed, andnotifies the forwarding execution information (transference executioninformation) to a mobile station 14.

In FIG. 1, when handover sequence is started, packets n−2 to n addressedto the mobile station are stored in a buffer BF of the handover sourcebase station 11 a and the packet n−2 is being transmitted to the mobilestation, but the transmission of this packets n−2 does not end. Duringexecution of the handover sequence, the handover target base station 11b sends information of the communication state on the handover targetbase station to the handover source base station 11 a along with an HOresponse message. When the communication state information is received,the handover source base station 11 a decides whether data transferencecan be executed depending on the communication state of the handovertarget base station 11 b, and attaches the transference executioninformation, which indicates whether data transference is executed ornot, to HO instruction and sends it to the mobile station 14.

FIG. 2 shows format example of HO response messages, including thecommunication state information. This message includes at least anidentifier ID and communication state information CST, as shown in (A)of FIG. 2. As the identifier, a new identifier which at least the mobilestation uses when it communicates with the handover target base station,may be included. And the address of the handover source base station 11a may be included as a destination address of this message and theaddress of the handover target base station 11 b may be included as thetransmission source address of this message. If the communication stateinformation is represented by X bits, the number of states that can bedefined is 2^(X).

As (B) of FIG. 2 shows, the communication state information CST hascongestion degree information according to the degree of congestion ofthe handover target base station 11 b. For example, if the congestiondegree information is represented by two bits, four congestion degreescan be represented (serious congestion, heavy congestion, lightcongestion, no congestion). On the other hand, the communication stateinformation CST may include buffer state information for notifyingresidual amount of memory of the handover target base station, as shownin (C) of FIG. 2. For example, the buffer state information isrepresented by 10 bits, and 1024 states can be represented. If the unitof this value is Kbytes, it is possible to represent that a maximum of1024 Kbytes of memory is secured for this mobile station. Further thecommunication state information CST may include packet transfer functioninstallation information in the handover target base station as shown in(D) of FIG. 2. This transfer function installation information isinformation to indicate whether the packet transfer function has beeninstalled or not. For example, in order to transfer packets from thehandover source base station 11 a to the handover target base station 11b, packet transfer protocol (packet transfer function) is required.Information on whether such a function has been installed or not can berepresented by 1 bit, and the installation of the function is notifiedto the handover source base station 11 a by this bit.

After the handover source base station 11 a receives the congestionstate information message, the possibility of data transference isjudged. For example, if “light congestion” is notified by the handovertarget base station 11 b, data transference is executed. If “heavycongestion” is notified by the handover target base station 11 b, datatransference is not executed. On the other hand, if the residual memoryamount of the handover target base station 11 b is an amount that cansufficiently store all the packets to be transferred, data transferenceis executed. If it is notified that the residual memory amount of thehandover target base station 11 b is “0 Kbyte”, then data transferenceis not executed. Further, if the handover target base station 11 b hasinstalled the packet transfer function, the data transference isexecuted. If it is notified that the packet transfer function is notinstalled, then data transference is not executed.

FIG. 3 is a block diagram of the base station, in which a buffer unit21, scheduler unit 22, transmission/reception unit 23 and control unit24 are shown.

The buffer unit 21 is a memory for storing packets that flow in from ahost station, and packets transferred from an adjacent base station(handover source base station). In FIG. 3, the base station physicallyhas two buffers, but may have one physical memory partitioned bysoftware. The scheduler unit 22 selects a mobile station with whichradio transmission is performed, from among a plurality of mobilestations in communication, extracts packets of this mobile stationstored in the buffer unit, and inputs them to the transmission/receptionunit 23. The transmission/reception unit 23 encodes and modulatespackets input from the scheduler, and transmits them by radio. Thetransmission/reception unit 23 also receives and demodulates controlsignals and various data which are sent from the mobile station.

The control unit 24 has a buffer management unit 24 a, HO control unit24 b and measurement information control unit 24 c. The buffermanagement unit 24 a manages various packets stored in the buffer 21. Ifdata transference is executed during handover, all the packets stored inthe buffer unit 21 are transferred to the handover target base station.On the other hand, if data transference is not executed even if handoveris generated, the buffer management unit 24 a controls such that all thepackets stored in the buffer unit 21 are erased. The HO control unit 24b executes the handover control described in FIG. 21, and themeasurement control unit 24 c collects various measurement informationwhich is sent from the mobile station, such as radio quality CQI(Channel Quality Information) of the mobile station.

FIG. 4 is a block diagram of the mobile station, in which atransmission/reception unit 31, buffer unit 32, reorder unit 33 andcontrol unit 34 are shown. The transmission/reception unit 31transmits/receives packets and control information to/from thetransmission/reception unit of the base station. If RLC PDU cannot beconstructed from the received lower layer packets, the buffer unit 32holds the packets until the RLC PDU is constructed, and if RLC PDU canbe constructed, the buffer unit 32 removes the header and transfers thepackets to the reorder unit 33 as RLC SDU (see FIG. 24). The reorderunit 33 has a function to rearrange RLC SDU (=PDCP PDU) in the order ofsequence numbers, and transfer it to the upper layer. If it is detectedthat the continuity of the sequence number of a RLC SDU (=PDCP PDU)ceases, the reorder unit 33 stores the subsequent RLC SDU (=PDCP PDU) inthe internal memory until the continuity of the sequence number isresumed by the received RLC SDU. If such RLC SDU (=PDCP PDU) does notarrive even if a predetermined time elapses, the reorder unit stops thereordering processing, and transfer all the stored PDCP PDU to the upperlayer.

The control unit 34 has a measurement control unit 34 a, reordermanagement unit 34 b and retransmission management unit 34 c. Themeasurement control unit 34 a measures various measurement informationto be sent to the base station. For example, radio quality CQI (ChannelQuality Information) of the mobile station is measured. The reordermanagement unit 34 b controls the reorder unit 33, and if the continuityof the sequence number of a PDCP PDU ceases, the reorder management unit34 b instructs the reorder unit 33 to store the PDCP PDU and wait forthe arrival of PDCP PDU by which the continuity of the sequence numbersis resumed. When a predetermined time to wait for the arrival of thepackets elapses, the reorder management unit 34 b instructs the reorderunit 33 to stop reordering, and also removes the header of all thestored PDCP PDUs, and transfers them to the upper layer as PDCP SDUs, sothat new PDCP PDUs can be received. During retransmission control time,the retransmission management unit 34 c sends the retransmission requestsignals to the base station via the transmission/reception unit 31 asshown by the dotted line.

If forwarding is executed in FIG. 1, the transmission of the packet n−2is stopped, and the packets n−2 to n are transferred to the handovertarget base station 11 b. If forwarding of the packets is not executed,on the other hand, the transmission of the packet n−2 is stopped, andthe packets n−2˜n are erased from the buffer unit BF. The handoversource base station 11 a attaches the transference execution informationto indicate whether data transfer (data forwarding) is executed, to anHO instruction message to the mobile station 14, and sends the HOinstruction message.

FIG. 5 is a flow chart depicting an operation of the handover targetbase station 11 b for executing a series of above mentioned operations.

In FIG. 5, when the handover control unit 24 b of the handover targetbase station 11 b receives an HO request (including mobile station IDand QoS information) from the handover source base station 11 a (step201), the handover control unit 24 b performs call acceptance controlbased on this information, and judges whether acceptance of the mobilestation is admitted (step 202). If not admitted, post-processing isperformed (step 211), and handover control ends.

If acceptance of the mobile station is admitted, on the other hand, thehandover control unit 24 b judges the communication state (congestedstate, buffer use state or function installed state) of the handovertarget base station 11 b (step 203), and returns the HO request responsemessage including the communication state information, which wasdescribed in FIG. 2, to the handover source base station 11 a (step204). Then the handover target base station 11 b stores the packetsforwarded from the handover source base station 11 a into the buffer 21in a case where the data transference is executed (step 124), andhereafter, the buffer unit 21 stores the packets forwarded from thehandover source base station 11 a.

If the handover control unit 24 b receives the HO completion report fromthe mobile station 4 in this state (step 206), the handover control unit24 b sends the HO completion report to the host station 12 (step 207).When the handover completion report is received, the host station 2switches the packet transmission path from the handover source basestation 11 a to the handover target base station 11 b, and returns withthe HO completion response to the handover target base station 11 b(step 208). When the HO completion response is received from the hoststation 12, the handover control unit 24 b of the handover target basestation 11 b instructs the scheduler 22 to start transmission of thepackets. Thereby the scheduler 22 starts transmission of the forwardedpackets to the mobile station 14 with priority, and then startstransmission of the packets received from the host station 12 to themobile station after sending the above packets (scheduling: step 209).In parallel with step 209, the handover control unit 24 b sends aresource release to the handover source base station 11 a (step 210),performs post-processing (step 211), and ends handover control.

The handover source base station 11 a notifies the mobile station 14 ofwhether data transference was executed or not using the HO instructionmessage, as mentioned above. FIG. 6 is a format example of the HOinstruction message, and this message includes at least the identifierID and transference execution information PH0. As the identifier, atleast a new identifier, which the mobile station 14 uses when itcommunicates with the handover target base station 11 b, is included. Anaddress of the mobile station 14 may be included as the destinationaddress of this message, and an address of the handover source basestation 11 a may be included in the transmission source address of thismessage. Also, in order to notify the transference executioninformation, at least one bit of information is included. For example,value “0” of this information indicates that data transference (packetforwarding) was executed, and value “1” of this information indicatesthat data transference was not executed. On the contrary, value “0” ofthis information may indicate that data transfer was not executed, andvalue “1” of this information may indicate that data transference wasexecuted.

FIG. 7 is a flow chart depicting a device operation of the handoversource base station for executing a series of the above mentionedoperations.

In FIG. 7, when the measurement control unit 24 a of a user of thehandover source base station 11 a receives the receive state informationfrom the mobile station 14 using Measurement Report (step 251), themeasurement control unit 24 a judges whether handover HO is requiredbased on this receive state information (step 252), and returns to thebeginning if handover is unnecessary.

If it is decided that the handover HO is necessary, the handover controlunit 24 b determines the handover target base station 11 b based on thecontent of Measurement Report, and sends the handover request to thishandover target base station 11 b (step 253).

Then the handover control unit 24 b receives an HO response messagetransmitted from the handover target base station 11 b (step 254),decides whether transfer of the packets (packet forwarding) is executedbased on the communication state information CST included in this HOresponse message (step 255), and if it is decided that the transferenceof the packets is not executed, the HO control unit 24 b enters thetransference execution information PH0 (=“1”: transference is notexecuted) in the HO instruction message, and sends it to the mobilestation (step 256), and instructs the buffer management unit 24 a toerase the packets remaining in the buffer 21 (step 257).

If it is decided that the transference of the packets is executed instep 255, on the other hand, the HO control unit 24 b enters thetransfer execution information PH0 (=“0”: transference is executed) inthe HO instruction message, and sends it to the mobile station 14 (step258), and instructs the buffer management unit 24 a to forward thepackets remaining in the buffer 21 to the handover target base station11 b (packet forwarding) (step 259). Thereby the buffer management unit24 a forwards the packets, which have not been sent to the mobilestation 14 and which remain in the buffer 21, to the handover targetbase station 11 b via the route indicated by the dotted line. Then theHO control unit 24 b receives the resource release message sent from thehandover target base station 11 b (step 260), and executes resourcerelease (step 261).

In FIG. 1, the mobile station 14 receives the transference executioninformation PH0 included in the HO instruction message, and startsreordering if the transference was executed. Specifically, in the casewhen the mobile station 14 received n+1 and later packets before thepackets n−2 to n, the mobile station 14 waits for the arrival of thepackets n−2 to n until a predetermined time elapses, withouttransferring the packet n+1 to the upper layer. If the transference wasnot executed, on the other hand, the mobile station 14 immediatelytransfers the n+1 and later packets to the upper layer, and prepares forreceiving the next packet without waiting for the arrival of the packetsn−2 to n.

FIG. 8 is a flow chart depicting an operation of the mobile station.

The measurement control unit 34 a of the mobile station 14 notifies thereceive state to the handover source base station 11 a using MeasurementReport (step 271). Then, the control unit 34 waits for an HO instructionmessage which is sent from the handover source base station 11 a, and ifthe HO instruction message is received (step 272), the control unit 34insures synchronization with the handover target base station 11 b byL1/L2 signaling (step 273), and sends the handover completion report tothe handover target base station 11 b if synchronization is insured(step 274). Then the control unit 34 checks whether data transference(data forwarding) was executed by referring to the transferenceexecution information PH0 of the HO instruction message received in step272 (step 275), and if data transference was not executed, the controlunit 34 performs a control to generate PDCP SDU using packets receivedfrom the handover target base station 11 b, and transfer it to the upperlayer, without executing reordering processing (step 276). If it isdetected that data transference was executed in step 275, on the otherhand, the reorder management unit 34 b controls the reorder unit 33, andexecutes the reordering processing (step 277).

FIG. 9 is a flow chart depicting the reordering processing of the mobilestation.

When the transmission/reception unit 31 receives lower layer packetsfrom the handover target base station 11 b, the reorder management unit34 b checks whether RLC PDU can be constructed (step 302), checkswhether a predetermined time has elapsed if it cannot be constructed(step 303), and stores these lower layer packets in the buffer 32 (step304) and executes processing after step 301 if the predetermined timehas not yet elapsed. If RLC PDU cannot be constructed even if thepredetermined time has elapsed after the lower layer packets arereceived, the lower layer packets are erased from the buffer (step 305).

If RLC PDU can be constructed using the lower layer packets received instep 302, on the other hand, the reorder management unit 34 b transfersthis RLC PDU to the reorder unit 33 as RLC SDU (step 306). If RLC SDU isreceived, the reorder unit 33 checks whether there is any missingsequence number and confirms the continuity of the sequence number (step307), and transfers the RLC SDU to the upper layer as the RDCP SDU ifsequence numbers are continuous without any missing (step 311). If thereis any missing sequence number, however, the reorder management unit 34b instructs the reorder unit 33 to stores RLC SDU (=PDCP PDU) (step308). Thereby the reorder unit 33 stores RLC SDU (=PDCP PDU) in theinternal memory and checks whether the continuity of the sequencenumbers is resumed by the received RLC SDU (step 309). If the continuityof the sequence numbers is resumed by the received RLC SDU, the reorderunit 33 transfers this RLC SDU to the upper layer as PDCP SDU, andtransfers the stored RLC SDU (=PDCP PDU) to the upper layer (step 311).

If RLC SDU by which the continuity of the sequence numbers is resumed,is not received in step 309, the reorder unit 33 monitors whether thepredetermined time has elapsed (step 310), repeats processing after step308 if not elapsed, or transfers stored PDCP PDU to the upper layer ifelapsed, even if the sequence numbers are not continuous (step 311).

As described above, according to the first embodiment, if packets whichremain in the handover source station are not forwarded to the handovertarget base station when handover control is executed (transference isnot executed), the mobile station does not have to execute reordering,even if the sequence numbers are not continuous, so unnecessary waittime is not generated, and delay time of data can be minimized andthroughput of the entire system can be improved.

(C) Second Embodiment

FIG. 10 is a diagram depicting a second embodiment, where a handoversource base station 11 a judges whether packet transfer (packetforwarding) is performed or not based on the type of communicationservice when the handover sequence is executed, and notifies theforwarding execution information (transference execution information) toa mobile station 14.

It is assumed that the handover source based station 11 a stores packetsof non-real-time traffic (NRT traffic) in FTP (File Transfer Protocol)communication or Web communication and packets of real-time traffic (RTtraffic) in VoIP communication and streaming respectively in differentbuffers BF11 and BR12, with different sequence numbers attached theretoand performs scheduling to transmit these packets, as shown in FIG. 10.It is also assumed that one of the packets m−2 to m constituting NRTtraffic and packets n−2 to n constituting RT traffic is in-transmissionand others are now stored in the buffers BF1, BF2 without beingtransmitted to the mobile station 14. In the second embodiment, the datatransference is executed based on the policy that the packets of NRTcommunication service are forwarded, and the packets of RT communicationservice are not forwarded. This is because of the followings. Forpackets of communication service that must be processed in highreal-time, such as packets of VoIP calls, it is preferable to discardthem without forwarding in order to transmit and receive voice withoutdelay. And for packets of low real-time and high QoS services it ispreferable to forwarding them to the handover target base station inorder to receive high quality data.

If the mobile station 14 which executes handover is performing RTcommunication, an HO control unit 24 b (see FIG. 3) of the handoversource base station 11 a decides not to forward the packet n−2 which iscurrently being transmitted, and packets n−1 to n which are currentlystored are not forwarded to the handover target base station 11 b whenthe handover sequence is executed based on the above mentioned packettransference policy. And the HO control unit 24 b includes atransference execution information PH0 indicating that forwarding wasnot performed in the HO instruction message, to notify the mobilestation 14. The message format in this case is the same as FIG. 6. Themobile station 14, which received the transference execution informationPH0, performs a similar operation as FIG. 8 and FIG. 9, as shown in thefirst embodiment, without executing the reordering processing.

If the mobile station 14 which executes handover is performing NRTcommunication, on the other hand, the HO control unit 24 b of thehandover source base station 11 a decides to forward the packet m−2which is currently being transferred, and packets m−1 to m which arecurrently stored are forwarded to the handover target base station 11 bwhen handover sequence is executed. Then transference executioninformation PH0 to notify that forwarding was executed is included inthe HO instruction message, and is sent to the mobile station 14. Themessage format in this case is the same as FIG. 6. Mobile station 14,which received the transference execution information PH0, performs anoperation the same as FIG. 8 and FIG. 9, and reordering processing isperformed as shown in the first embodiment.

FIG. 11 is a flow chart depicting an operation of the handover sourcebase station 11 a for executing the above mentioned series ofoperations, in which the same steps as the flow chart of the firstembodiment in FIG. 7 are denoted with the same numbers. The differenceis step 255′, in which it is determined whether the transference of thepackets (packet forwarding) is executed or not depending on whether thecommunication service is NRT communication service or RT communicationservice. If the communication service is RT communication service, an HOcontrol unit 24 b decides that forwarding is not executed, enters thetransference execution information PH0 (=“1”: transference is notexecuted) in an HO instruction message, and sends it to the mobilestation (step 256), and instructs a buffer management unit 24 b to erasethe packets remaining in a buffer 21 (step 257).

If it is decided that the transference of the packets is executed instep 255′, on the other hand, the HO control unit 24 b enters thetransference execution information PH0 (=“0”: transference is executed)in the HO instruction message, and sends it to the mobile station 14(step 258), and instructs the buffer management unit 24 a to forward thepackets (transfer the packets) remaining in the buffer 21 to thehandover target base station 11 b (step 259).

Variant Form

In the second embodiment, transference is not executed if thecommunication service is RT communication service, but in some cases,packets may or may not be forwarded depending on the setting even if thecommunication service is RT communication service (e.g. VoIP calls).This variant form is for supporting such a case, and FIG. 12 is a flowchart depicting an operation of the handover source base station 11 a ofthe variant form, in which the same steps as the processing flow of thefirst embodiment in FIG. 11 are denoted with the same numbers. Thedifference is the existence of step 265, in which it is checked whetherthe setting is to forward packets or not if the communication service isRT communication service, and if not, the HO control unit 24 b decidesthat forwarding is not executed, enters the transference executioninformation PH0 (=“0”: transference is not executed) in the HOinstruction message, and sends it to the mobile station (step 256), andinstructs the buffer management unit 24 b to erase the packets remainingin the buffer 21 (step 257).

If the setting is to forward packets in step 265, on the other hand, theHO control unit 24 b enters the transference execution information PH0(=“0”: transference is executed) in the HO instruction message, andsends it to the mobile station 14 (step 258), and instructs the buffermanagement unit 24 a to forward the packets (transfers the packets)remaining in the buffer 21 to the handover target base station 11 b(step 259).

FIG. 13 is a flow chart depicting the reordering processing in themobile station of the variant form, in which the same steps as theprocessing flow of the first embodiment in FIG. 9 are denoted with thesame numbers, and the difference is that the time to continue reorderingprocessing is switched depending on whether the communication service isVoIP calling or not. In other words, it is checked whether the call typeis a VoIP call (step 312) if RLC SDU of which sequence numbers arecontinuous is not received in step 309, and if not VoIP calling, thecontrol unit 34 monitors whether the predetermined first setup time TS1has elapsed (step 310), repeats the processing after step 308 if notelapsed, or transfers the stored PDCP PDU to the upper layer if thesetup time has elapsed, even if the sequence numbers are not continuous(step 311). If the call type is VoIP calling in step 312, the controlunit 34 monitors whether a second setup time TS2 (<TS1) has elapsed(step 313), repeats the processing after step 308 if not elapsed, ortransfers the stored PDCP PDU to the upper layer if the setup time haselapsed, even if the sequence numbers are not continuous (step 311).

According to the above mentioned variant form, the quality of VoIP canbe improved by forwarding packets even in the case of VoIP calling, anddelay time due to reordering processing can be decreased by decreasingthe setup time TS2.

(D) Third Embodiment

The third embodiment is an embodiment to determine whether packettransfer (packet forwarding) is possible depending on whether a handoversource base station has a packet transfer function or not, notifyingthis execution information to a mobile station. FIG. 14 is a blockdiagram depicting a base station in which a buffer management unit has asimplified configuration without a packet transfer function, and thesame portions as the base station of the first embodiment in FIG. 3 aredenoted with the same symbols. The difference is that the buffermanagement unit 24 a′ has a simplified configuration, and does not havea function to forward packets to the handover target base station 11 bor a packet transfer scheduler function.

FIG. 15 is a flow chart depicting an operation of the handover sourcebase station, in which the same steps as the flow chart of the firstembodiment in FIG. 7 are denoted with the same numbers. The differenceis that it is determined whether the transference of the packets (packetforwarding) is executed or not depending on whether the forwardingfunction and the packet transfer scheduler function are installed in thehandover source base station 11 a in step 255″. If these functions arenot installed, an HO control unit 24 b decides that forwarding is notexecuted, enters the transference execution information PH0 (=“1”:transference is not executed) in an HO instruction message, and sends itto the mobile station (step 256), and instructs a buffer management unit24 b to erase the packets remaining in a buffer 21 (step 257).

If the forwarding function and the packet transfer scheduler functionare installed in step 255″, on the other hand, the HO control unit 24 bdecides to execute the transfer of the packets, enters the transferenceexecution information PH0 (=“0”: transference is executed) in the HOinstruction message, and sends it to the mobile station 14 (step 258),and instructs the buffer management unit 24 a to forward the packets(transfer the packets) remaining in the buffer 21 to the handover targetbase station 11 b (step 259).

(E) Fourth Embodiment

In the above embodiments, the PDCP layer is installed in aGW 12, whichis a host station, and the RLC layer and lower layer are installed inthe base station 11 as shown in (A) of FIG. 16, but as (B) of FIG. 16shows, a system may be constructed such that the entire PDCP layer, RLClayer and lower layer are installed in the base station 11, and onlysimple functions, such as packet routing function, may be installed inaGW 12. In the system configuration of (B) of FIG. 16, a sequence numberattaching function may be provided to the base station. FIG. 17 is adiagram depicting forwarding when handover is generated in such asystem.

Three packets are sent from a host station 12 to a handover source basestation 11 a, and packet numbers n−2, n−1 and n are attached in the basestation. It is assumed that handover is started in a state when thesepackets n−2, n−1 and n remain in a buffer without being sent to themobile station. If these packets n−2, n−1 and n are forwarded duringexecution of the handover sequence, these packets are transferred fromthe handover source base station 11 a to the handover target basestation 11 b as shown in FIG. 17. During this forwarding, packets may besent from the host station 12 to the handover target base station 11 b.In such a case, the handover target base station 11 b cannot attachcorrect sequence numbers to the packets received from the host station12. Therefore when the packets n−2, n−1 and n, which have not been sentto the mobile station, are forwarded to the handover target base station11 b, the handover source base station 11 a also forwards the sequencenumbers to the handover target base station. In other words, thesequence number n+1 to be attached to the packet, which is sent from thehost station 12 to the handover target base station 11 b is forwarded.Thereby the control unit 24 of the handover target base station 11 b canattach the sequence number n+1 to the packet exactly, even if packetsare sent from the host station 12 to the handover target base station 11b during forwarding.

In the case of the fourth embodiment as well, the transference executioninformation PH0, to indicate whether forwarding was executed or not, canbe included in the HO instruction message and sent to the mobilestation, whereby reordering processing can be omitted when forwarding isnot performed.

FIG. 18 is a diagram depicting the handover procedure of the fourthembodiment, in which the same procedures as the diagram depicting thehandover procedures in FIG. 21, are denoted with the same numbers. Thedifference is that the sequence numbers SN are transferred from thehandover source base station 11 a to the handover target base station 11b before the packet transfer (packet forwarding).

According to the fourth embodiment, sequence numbers are forwardedduring handover even if the sequence numbers are assigned to the packetsin the handover source base station, so correct sequence numbers can beattached to the packets in the handover target base station, and ifforwarding is not executed, reordering processing can be omitted in themobile station.

Advantage of the Embodiments

According to the present invention, when packets remaining in thehandover source base station are not forwarded to the handover targetbase station (when transference is not executed) at the execution ofhandover control, the mobile station does not have to executereordering, even if the sequence numbers are not continuous, sounnecessary wait time is not generated, delay time of data is minimized,and throughput of the entire system can be improved.

Furthermore, according to the present invention, the mobile station doesnot have to execute reordering, even if the forwarding is not executeddepending on the communication state (congested state, buffer emptystate, function installed state) and type of communication service, sounnecessary wait time is not generated, delay time of data is minimized,and throughput of the entire system can be improved.

Furthermore, according to the present invention, packets are forwardedduring handover even in the case of VoIP calling, so the quality of VoIPcan be improved, and delay time due to reordering processing can bedecreased in the mobile station by decreasing the duration of reorderingprocessing.

Furthermore, according to the present invention, sequence numbers areforwarded during handover even if the sequence numbers are attached topackets in the handover source base station, so correct sequence numberscan be attached to the packets in the handover target base station, andreordering processing can be omitted in the mobile station if forwardingis not performed.

1. A reordering method for sending packets, to which numbers indicatingsequence are attached, from a base station to a mobile station, andrearranging the packets in order of the sequence number in the mobilestation, the reordering method comprising: when a handover source basestation transfers packets which have not been sent to the mobilestation, out of packets received from a host station, to a handovertarget base station, and the handover target base station sends thepackets to the mobile station, sending data from the source base stationto the mobile station to notify that transference of the packets fromthe source base station to the target station has been executed at theexecution of handover sequence; sending data from the source basestation to the mobile station to notify that transference of the packetsis not executed at the execution of the handover sequence, when thepackets are not transferred to the handover target base station; and atthe mobile station judging whether transference of the packets isexecuted with reference to said data, and executing reordering of thepackets received from the handover target base station when thetransference of the packets has been executed wherein the mobile stationdoes not execute reordering of the received packets when thetransference of the packets has not been executed.
 2. The reorderingmethod according to claim 1, further comprising a step of: ending thereordering processing at the mobile station when packets havingcontinuous sequence numbers are not received even if reorderingprocessing of the received packets is executed for a predetermined time.3. The reordering method according to claim 1, wherein packets which arebeing transmitted when execution of handover sequence starts are packetsfor the transference.
 4. The reordering method according to claim 1,further comprising: deciding by the handover source base station whetherthe packets, which have not been sent to the mobile station, are to betransferred to the handover target base station or to be erased, byreferring to communication state information of the handover target basestation that is received from the handover target base station at theexecution of the handover sequence.
 5. The reordering method accordingto claim 4, wherein the handover source base station refers to thecommunication state information, and transfers the packets, which havenot been sent to the mobile station, to the handover target base stationwhen the handover target base station is not in a congestion state, anderases the packets without transferring them to the handover target basestation when the handover target base station is in a congestion state.6. The reordering method according to claim 4, wherein the handoversource base station refers to the communication state information, andtransfers the packets, which have not been sent to the mobile station,to the handover target base station when a buffer of the handover targetbase station has sufficient space, and erases the packets withouttransferring them to the handover target base station when the bufferdoes not have sufficient space.
 7. The reordering method according toclaim 4, wherein the handover source base station refers to thecommunication state information, and transfers the packets, which havenot been sent to the mobile station, to the handover target base stationwhen the handover target base station has a function to receive packetsfrom the handover source base station and send them to the mobilestation, and erases the packets without transferring them to thehandover target base station when the handover target base station doesnot have the function.
 8. The reordering method according to claim 1,further comprising: deciding by the handover source base station whetherthe packets, which have not been sent to the mobile station, are to betransferred to the handover target base station or to be eraseddepending on the type of communication service.
 9. The reordering methodaccording to claim 8, wherein the handover source base station transfersthe packets, which have not been sent to the mobile station, to thehandover target base station if the type of communication service isnon-real-time communication, and erases the packets without transferringthem to the handover target base station if the type of communicationservice is real-time communication.
 10. The reordering method accordingto claim 2, wherein the mobile station changes the length of thepredetermined time to continue the reordering processing depending onthe type of communication service.
 11. The reordering method accordingto claim 1, wherein the handover source base station transfers thepackets, which have not been sent to the mobile station, to the handovertarget base station when the handover source base station has a functionto transfer the packets to the handover target base station, and erasesthe packets without transferring them to the handover target basestation when the handover source base station does not have thefunction.
 12. The reordering method according to claim 1, furthercomprising: in a case where the base station attaches sequence numbersto the packets to indicate the sequence of the packets, transfers thesequence numbers to the handover target base station when the handoversource base station transfers the packets, which have not been sent tothe mobile station, to the handover target base station.
 13. Acommunication system for sending packets, to which numbers indicatingsequence are attached, from a base station to a mobile station, andrearranging the packets in order of the sequence number in the mobilestation, comprising: a handover source base station, a handover targetbase station and a mobile station wherein the handover source basestation having: a buffer which stores packets received from a hoststation; a packet transmission unit which sends the packets stored inthe buffer to the mobile station; and a control unit which sends data tonotify that transference of the packets has been executed to the mobilestation, when the handover source base station transfers the packets,which have not been sent to the mobile station, to the handover targetbase station at the execution of handover sequence, and sends data tonotify that transference of the packets is not executed to the mobilestation, when the packets are not transferred to the handover targetbase station; the handover target base station having: a buffer whichstores packets received from the handover source base station andpackets received from the host station at the execution of handoversequence; a control unit which controls such that the packets receivedfrom the handover source base station are sent to the mobile stationpreferentially; and a transmission unit which sends packets to themobile station; and the mobile station having: a buffer which storespackets received from the base station; and a control unit whichexecutes reordering of the received packets or does not executereordering of the received packets based on said data received from thehandover source base station and indicating whether transference of thepackets has been executed.
 14. The communication system according toclaim 13, wherein the control unit of the mobile station ends reorderingprocessing when packets having continuous numbers are not received evenif reordering processing of the received packets is executed for apredetermined time.
 15. The communication system according to claim 13,wherein the control unit of the handover source base station decideswhether the packets, which have not been sent to the mobile station, areto be transferred to the handover target base station or to be erased,by referring to communication state information of the handover targetbase station that is received from the handover target base station atthe execution of the handover sequence.
 16. The communication systemaccording to claim 13, wherein the control unit of the handover sourcebase station decides whether the packets, which have not been sent tothe mobile station, are to be transferred to the handover target basestation or to be erased depending on the type of communication service.17. The communication system according to claim 13, wherein the controlunit of the handover source base station transfers the packets, whichhave not been sent to the mobile station, to the handover target basestation when the control unit has a function to transfer the packets tothe handover target base station, and erases the packets withouttransferring them to the handover target base station when the controlunit does not have the function.
 18. The communication system accordingto claim 13, wherein in a case where the base station attaches sequencenumbers to the packets to indicate the sequence of the packets, thecontrol unit of the handover source base station transfers the sequencenumbers to the handover target base station when the handover sourcebase station transfers the packets, which have not been sent to mobilestation, to the handover target base station.
 19. A base station in acommunication system for sending packets, to which numbers indicatingsequence are attached, from a base station to a mobile station andrearranging the packets in order of the sequence number in the mobilestation, comprising: a buffer which stores packets received from a hoststation and packets received from a handover source base station at theexecution of handover sequence; a packet transmission unit which sendspackets stored in the buffer to the mobile station; a handover controlunit which decides whether packets, which have not been sent to themobile station, are to be transferred to a handover target base stationat the execution of handover sequence or not, transfers the packets tothe handover target base station and data to notify that transference ofthe packets has been executed to the mobile station if the packets areto be transferred to the handover target base station, and sends thedata to notify that transference of the packets is not executed to themobile station if the packets are not to be transferred to the handovertarget base station; and transmission control unit which sends thepackets received by the handover source base station to the mobilestation with priority over the packets received from the host station atthe execution of handover sequence.
 20. The base station according toclaim 19, wherein the handover control unit decides whether the packets,which have not been sent to the mobile station, are to be transferred tothe handover target base station or to be erased, by referring tocommunication state information of the handover target base station thatis received from the handover target base station at the execution ofthe handover sequence.
 21. The base station according to claim 19,wherein the handover control unit decides whether the packets, whichhave not been sent to the mobile station, are to be transferred to thehandover target base station or to be erased depending on the type ofcommunication service.
 22. The base station according to claim 19,wherein the handover control unit transfers the packets, which have notbeen sent to the mobile station, to the handover target base stationwhen the handover control unit has a function to transfer the packets tothe handover target base station, and erases the packets withouttransferring them to the handover target base station when the handovercontrol unit does not have the function.
 23. The base station accordingto claim 19, wherein in a case where the base station attaches sequencenumbers to the packets to indicate the sequence of the packets, thehandover control unit of the handover source base station transfers thesequence numbers to the handover target base station when it transfersthe packets, which have not been sent to the mobile station, to thehandover target base station.
 24. A mobile station in a communicationsystem for sending packets, to which numbers indicating sequence areattached, from a base station to a mobile station and rearranging thepackets in order of the sequence number in the mobile station,comprising: a buffer which stores packets received from the basestation; and a control unit which decides whether reordering of thepackets received from a handover target base station is to be executedor not based on data received from a handover source base station at theexecution of the handover sequence and indicating whether transferenceof the packets has been executed.
 25. The mobile station according toclaim 24, wherein the control unit ends reordering processing whenpackets having continuous numbers are not received even if reorderingprocessing of the received packets is executed for a predetermined time.